I feel your pain. I've been trying to overcome this issue for a couple of years now. Lost so much business because of it since managed payments started. We have noticed this extra complication that mite save some confusion though; the old "Request total" option (hard enough for buyers to find on a laptop) doesn't appear AT ALL when customers buy on their phones.

Unfortunately this pretty-much forces buyers onto listings with blanket freight charges included in the price and guess-who gets a bigger fee.